The skeletal muscle fibers of vertebrate and invertebrate organisms possess a remarkable plasticity, being one of the few differentiated cells in the adult that is able to substantially alter its size and biochemical properties in response to a variety of physiological and pathological conditions. In the skeletal muscles of vertebrate animals, the alteration of contractile properties is based on the transformation of muscle fibers by the coordinated expression of fiber-type-specific isoforms of myofibrillar proteins. Although avian and mammalian models have contributed greatly to our understanding of the molecular basis of muscle plasticity, we still know very little about the early events which trigger fiber transformation. We propose to develop a crustacean model, in which transformation can be predicted, to investigate these early events. Fibers in the claw closer muscles of the American lobster, Homarus americanus, undergo a developmentally-regulated transformation as the isomorphic claws of larvae and juveniles differentiate into the cutter and crusher claws of the adult. This transformation occurs at the boundary between the fast- and slow-fiber regions, and thus the transformation of a specific fiber is determined by its position relative to the border between the two fiber populations. Our long-term objective is to identify factors that induce and coordinate the expression of myofibrillar proteins during transformation. The overall goal of this proposal is to construct probes and develop essential experimental methods to measure the expression of fast and slow isoforms of myosin heavy chain (HC). The specific aims are to (1) isolate a cDNA encoding myosin HC from lobster slow-twitch muscle, (2) characterize the cDNAs encoding myosin HCs, and (3) quantify the expression of myosin HC in adult muscles. We have already isolated a fast myosin HC cDNA by screening an expression library with an antibody to the HC. We will use the same approach to isolate a slow myosin HC message from a cDNA library of crusher claw muscle. Isolated clones are sequenced to identify nucleotide sequences unique to fast or slow myosin HC messages. This will provide information ii necessary for the synthesis of hybridization probes, which are used for RNA blot analysis and in situ hybridization to compare myosin HC isoform expression and histochemical properties of adult claw and abdominal muscles.